Transmission line circuit



April 1955 N. E. LINDENBLAD 2,706,243

TRANSMISSION LINE CIRCUIT Filed Feb. 27, 1943 2 Sheets-Sheet! 7'0ANTENNA INVENTOR M45 5 [mos/vam ATTORNEY United States PatentTRANSMISSION LINE CIRCUIT Nils E. Lindenblad, Rocky Point, N. Y.,assignor to Radio Corporation of America, a corporation of DelawareApplication February 27, 1943, Serial No. 477,435

Claims. (Cl. 250-13) The present invention relates to high frequencyfilter systems for connecting one or more'transmitters and receivers toa common antenna system and, furthermore, to such systems for use inpulse transmitting and receiving systems for non-signaling purposes.

An object of the present invention is the provision of a filter systemfor enabling the selective connection of a transmitter and a receiver toa common antenna system without requiring the use of mechanicalswitching.

Another object of the present invention is the provision of a filternetwork, the operating characteristics of which are controlled by thepower applied thereto.

A further object of the present invention is the provision of a filtersystem for pulse transmitting and receiving systems.

Still a further object of the present invention is the provision of anantenna filter system for pulse transmitting and receiving equipmentwhich utilizes no moving parts or mechanical switches for diverting theout going and incoming pulses to the appropriate transducer equipment.

The foregoing objects, and others which may appear from the followingdetailed description, are attained in accordance with the principles ofthe present invention by providing a filter system between an antennaand receiver and a transmitter, said system utilizing a plurality ofresonant structures acting as filter elements. The structures may beempty resonant chambers or cavity resonators, or they may take the formof resonant line filters, and it should be clearly understood thathereafter in referring to resonant structures it is intended to includeany and all of the above mentioned types, whether or not actual lumpedor distributed circuit elements are employed. The filter elements are soplaced and are so connected to the antenna transmitter and receiverthrough electrical discontinuities that when signaling potentials areapplied to the system from the transmitter the electricaldiscontinuities are caused to break down and become conducting. Thefilter sections then cause the energy from the transmitter to beconducted to the antenna but prevent its application to the receiver.When the transmitter ceases operation the electrical discontinuities arerestored and the signals picked up by the antenna are applied to thereceiver. The electrical discontinuities are so placed that no receivedenergy is Wasted in the transmitter.

The present invention will be more completely understood by reference tothe following detailed description, which is accompanied by a drawing,in which Figure 1 illustrates in partial section an embodiment of thepresent invention, while Figures 2, 3 and 4 each illustratemodifications thereof. In Figure 1, represents in fragmentarycross-section a portion of a radio transmitter which may be of themagnetron type. This type of transmitter is particularly desirable whenoperating at ultra high frequencies though, of course, if it is desiredto use the present invention with lower frequency waves a transmitter ofsome more conventional type may be used. Coupled to the transmitter 10through the intermediary of a coupling loop 14 is one end of atransmission line 11 having an outer shell 13 and a concentricallylocated inner conductor 12. The other end of transmission line 11 isconnected to radiating means, as indicated by the label To antenna. Theinner conductor 12 has an electrical discontinuity 15 therein, thepurpose of which will be more fully deice scribed later. On the side ofthe electrical discontinuity 15, remote from the transmitter 10, isconnected a struc ture resonant to the operating frequency; saidstructure may be in the form of a hollow closed cylinder or shell 33having an overall length from one end wall 34 to the other 35 ofsubstantially a half of the operating wavelength. Through wall 34 of thecylinder 33 passes a second transmission line 23 having an outer shell32 and an inner concentrically arranged conductor 22. The exterior endof transmission line 23 is coupled to a suitable receiver of highfrequency energy. The outer conductor 32 of transmission line 23 extendssubstantially to the middle of cylinder 33 and at that point isconnected to the inner conductor 12 of transmission line 11. The innerconductor 22 of the transmission line 23 is carried on as anextension22' to the wall 35 of cylinder 33 and is separated therefromonly by an electrical discontinuity 25. Electrical discontinuities 15,25 may be actual breaks in the conductors forming spark gaps or gasdischarge tubes may be employed in their place if desired.

In the operation of the structure described, the operation oftransmitter 10 causes gap 15 to break down very easily thus energizingthe end of the shell 32 of the receiver transmission line 23. Theelectrical distance from the free end of shell 32 to a point directlyopposite, in shell 33 being a half wavelength, the end of shell 32presents a high impedance to the energy from transmitter 10. Some energybeing transferred to conductor 22, gap 25 breaks down and the quarterwave extension 22' also presents a high impedance to the energy at themouth of shell 32. Thus there is no effective potential differencebetween the conductors of transmission line 23 and very little energypasses to the receiver. The major portion of the energy from thetransmitter therefore passes on along transmission line 11 to theantenna. When the transmitter ceases operation the electricaldiscontinuities in each of the gaps 15 and 25 are restored. The gaps donot break down when a received signal arrives. The end of extension 22'adjacent wall 35 is thus substantially free. At the mouth of shell 32the resultant eiiect is as though the center conductor 22 oftransmission line 23 were grounded at the mouth of shell 32. Since theend of shell 32 is free to swing with respect to ground the receivedenergy passes to the receiver. The gap 15 in line 12 from thetransmitter blocks the energy from going in that direction.

It will thus be apparent that in operation substantially all of theenergy from the transmitter will be applied to the antenna while whenthe transmitter is not in operation and received signals come from theantenna to the filter network substantially all of the received energywill be applied to the receiver, none being wasted in the transmittercircuits.

The modification of the present invention illustrated in Figure 2 ingeneral operates in the same way as described above with reference toFigure 1. However, instead of, as in Figure 1,-providing a series gap 15in the line to the transmitter 10 in Figure 2 a supplemental trapcircuit including shell 43 and central conductor 42 is provided. Thecentral conductor 42 is connected at its other end to the end of shell43 through a spark gap or other electrical discontinuity 45. It will benoted that the length of conductor 42 is a quarter wavelength at theoperating frequency and the length of conductor 12 between the point ofconnection of conductor 42 and shell 32 is also a quarter wavelength.Now, when transmitter 10 supplies energy to the transmission line 13,gap 45 breaks down thus effectively grounding the conductor 42 at itsend to shell 43. It therefore presents at its point of connection toconductor 12 a high impedance to the high frequency energy fromtransmitter 10 and substantially no energy is lost at that point. Thetransmitted energy is then fed to the antenna along conductor 12 asdescribed with reference to Figure 1. However, when receiving signalsneither gap 35 or gap 45 breaks down. Gap 35 acts in the same way asdescribed before. Gap 45 causes an extremely low impedance to the energyto be presented at the junction of conductor 42 and conductor 12 sinceconductor 42 is now an open-ended quarter wave conductor. Due to thequarter wavelength spacing between the junction of conductors 42 and 32with conductor 12 a high impedance is presented at the junction ofconductors 12 and 32. Conductor 22 being efiectively grounded at themouth of shell 32 and the said mouth being effectively at high impedanceabove ground. all of the received energy will be applied to the receiverthrough transmission line 23. This modification has the advantage overthe previously described modification in that none of the spark gaps arerequired to carry large amounts of current. It is only necessary thatthey carry sufficient energy to break down the gaps.

The modification in Figure 3 illustrates the use of cavity resonatorsfor providing the switching functions between the transmitter andreceiver and antenna. In this modification there is provided a cavityresonator 53 operating as a structure resonant to the desired frequency.The resonator is preferably so dimensioned that the oscillationstherewithin are of the lowest mode, that is, with the electric fieldparallel to the axis and with a transverse circular magnetic field. Herethe energy from the transmitter. Breaks down gap 56 and due to thecurrent flow along conductor 12 excites the interior of the cavity toresonance. Conductor 62 from the antenna passing through the cavity 53and linking the lines of force within the cavity in the same way asconductor 12 picks up the energy from the transmitter and applies it tothe antenna. Sufficient energy is picked up by extension 54 by innerconductor 22 of the receiver transmission line so that gap 52 breaksdown. Due to conductor 54 within shell 55 being substantially a quarterwave in length, point X on the conductor 54 appears at a very highimpedance. Thus substantially no current can flow along conductor 22 andno energy is supplied to the receiver. When the transmitter ceasesoperation and incoming signals are applied to conductor 62 the interiorof cavity 53 is again excited. Due to the fact that gap 56 is now openno current can flow along conductor 12 and no energy is applied to thetransmitter. However, gap 52 is also open which causes point X onconductor 54 to appear as a very low impedance. Thus current may flowalong conductor 22 and the received energy may be applied to thereceiver.

The operation of the modification shown in Figure 4 is the same as thatshown in Figure 3, the only substantial distinction being that thetransmitter and receiver transmission lines 13 and 23 both enter thecavity 53 from the same side. This is of some convenience in assuringthe most direct connections between the filter network, the transmitter,the receiver and the antenna.

While I have illustrated several embodiments of the present invention,it should be clearly understood that it is not limited thereto sincemany further modifications may be made in the several elements employedand in their arrangement and it is therefore contemplated by theappended claims to cover any such modifications as fall within thespirit and scope of the invention.

I claim:

1. A filter element including a first conductor, a second conductorparallel thereto and of substantially equal length, a third conductorparallel to said first two of one half of the length of each of said twoand connected to said first conductor at one end thereof, a twoconductor transmission line coupled at an intermediate point to saidfirst conductor and to said third conductor at its free end, a dischargepath connecting said first and third conductors at the other end of saidfirst conductor, a transmitter coupled through a discharge path to saidtransmission line and receiver means coupled across said second andthird conductor.

2. A filter system including a resonant chamber having a pair ofopposite walls spaced apart a distance equal to a half of the operatingwavelength, a first transmission line having an outer shell and an innerconductor, the outer shell of said transmission line passing through oneof the opposing walls of said chamber and extending into said chamber adistance equal to one quarter of the operating wavelength, the innerconductor of said transmission line being coupled to the other of saidopposing walls through a discharge path, a second similar transmissionline having its inner conductor entering said chamber and connected tothe end of the outer shell of said first transmission line, the outershell of said second transmission line being connected to the casing ofsaid resonant chamber and a source of high frequency energy connected tosaid second transmission line in said chamber through a second dischargepath.

3. A junction between a plurality of transmission lines, one of saidlines being associated with a source of high frequency energy, saidjunction including a chamber resonant to said energy, a conductor ofsaid one transmission line having an electrical discontinuity thereinand passing into said chamber, another of said transmissing lines havingassociated therewith a receiver, a conductor of said other transmissionline extending into said chamber and coupled thereto through anelectrical discontinuity, the extending portion of said conductor havingsuch length that when said discontinuity is closed said othertransmission line is not coupled to said chamber, the last of saidtransmission lines being associated with an antenna and coupled to theinterior of said chamber.

4. A junction between a plurality of transmission lines, one of saidlines being associated with a source of high frequency energy. saidjunction including a chamber resonant to said energy, a conductor ofsaid one transmission line having a discharge path therein and passinginto said chamber, another of said transmission lines having associatedtherewith a receiver, a conductor of said other transmission lineextending into said chamber and connected thereto through a dischargepath, the extending portion of said conductor having a length equal toone quarter of the operating wavelength such that when said dischargepath is closed said other transmission line is not coupled to saidchamber, the last of said transmission lines being associated with anantenna and coupled to the interior of said chamber.

5. A junction between a plurality of transmission lines, one of saidlines being associated with a source of high frequency energy. saidjunction including chamber resonant to said energy, a conductor of saidone transmission line having a spark gap therein and passing into saidchamber, another of said transmission lines having associated therewitha receiver, a conductor of said other transmission line extending intosaid chamber and connected thereto through a spark gap, the extendingportion of said conductor having a length equal to one quarter of theoperating wavelength such that when said spark gap is closed said othertransmission line is not coupled to said chamber, the last of saidtransmission lines being associated with an antenna and coupled to theinterior of said chamber.

Hansen et al. Apr. 28, 1942 Carter Dec. 21, 1943

